The present invention relates to an automatic landing method and device for an aircraft, in particular a transport airplane, on a landing runway having a strong slope being higher than a predetermined value.
In the scope of the present invention, the expression strong slope (of a landing runway) means a slope which is higher (in absolute value) than a predetermined value, preferably 0.8%, and more precisely higher than the usual certification values for a current automatic piloting system.
On the present airplanes, an onboard automatic landing system can be developed so as to allow a landing in a bad visibility condition. To perform such a landing with no visibility (or with a very weak visibility), it is necessary to have available ground facilities (of the usual “ILS CAT II” or “ILS CAT III” type) which are quite expensive.
Furthermore, new guiding means of the GPS type with a regional increase (for example either of the WAAS type in Northern America or of the AGNOS type in Europe) are expanding. Such means which do not need any ground facilities on each airport present precision levels which make their use conceivable to perform an automatic landing. Even if such means do not allow the visibility minima to be reduced, an automatic landing can be preferable to a manual landing when the external conditions become unfavorable (crosswind, turbulence, downwind, front sun, night condition for example).
On the airplanes provided with a usual automatic landing function, the automatic piloting system possesses a flare-out control law allowing the vertical speed of the airplane to be reduced so as to obtain:                an impact vertical speed that is weak (typically −2 feet per second); and        an impact point that is close to the theoretical touch point (typically 400 m downstream from the runway threshold).        
Indeed, a high impact vertical speed can lead to a passenger discomfort and, if it goes beyond extreme values, to structural limit values being exceeded, for which the airplane has been designed.
Moreover, if the actual impact point is too far from the theoretical impact point, the remaining runway distance for braking on the airplane decreases and, in extreme cases, can lead to a turn-off of the airplane from the landing taxiway.
Such two main parameters (impact vertical speed and impact point) must thus be controlled, and the aeronautical regulations impose to an automatic piloting system provided with an automatic landing function a statistical performance demonstration of the system, so as to show that the probability to exceed extreme values remains included in an acceptable proportion, whatever the external conditions.
Usually, the flare represents the part of the approach trajectory immediately preceding the contact with the ground, during which a straightening procedure of the airplane is performed.
The present automatic piloting systems are certified on slightly inclined airfields (for example +/−0.8%, what allows in practice the whole airfields equipped with CAT II and CAT III type facilities to be covered, for which an automatic landing system is required.
On the contrary, in a manual piloting, an airplane is certified to land on airfields presenting higher slopes (for example +/−2%).
On airfields with high slope, the automatic landing function (of an automatic piloting system) cannot be presently used.
Indeed, usually, an automatic piloting system uses for the flare phase, in order to be positioned in altitude with respect to the runway, a radio altimeter that measures the height with respect to the airfield, directly under the airplane.
In the usual systems, the radio altimeter is used:                on the one side, for starting the flare when a given height with respect to the airfield (which can be adapted depending on the airplane speed and can be located before or after the runway threshold) is reached by the airplane; and        on the other side, to estimate the runway slope and adapt the airplane trajectory therefor.        
The flare phase is generally very short (typically 7 s). Consequently, the automatic piloting system has not much time available to perform the corrections. The setting of the flare law goes thru an a priori knowledge of the procedure to follow (for instance by using mass and ground speed) to adapt the starting height and a pre-control (initial value of the nose up command to apply to the aircraft) so as to correctly initiate the trajectory change being required during such phase.
On airfields with high slope, the problems to be solved are as follows:                on a climbing runway, the flare starting must be anticipated, frequently before the runway threshold and the pre-control must be strong, otherwise the ground impact speed will be very high; and        on a descending runway, the flare starting must be delayed, frequently well after the runway threshold and the pre-control must be very weak, otherwise the impact area of the airplane wheels is very distant from the runway threshold in the case where the slope is unfavorable for the airplane braking.        
On the airfields with strong slope, it is thus necessary to know the runway slope before even the flare starting, but also before the runway threshold, including on climbing runways. Now, the available radio altimeter information, which allows the runway slope to be determined, can only be measured by the radio altimeter under the airplane (and not ahead of the airplane).
Furthermore, it is hardly conceivable to use the slope information before the runway threshold to extrapolate the runway slope, since there is no guarantee of continuity.
Consequently, a usual automatic piloting system is not in a position to perform an automatic landing on a runway with strong slope with the means being available, namely a radio altimeter only.
The present invention aims at remedying such drawbacks. It relates to a method to perform an automatic landing of an aircraft on a landing runway presenting a strong slope, which is higher than a predetermined value and preferably higher than the usual certification values for a current automatic piloting system.
With this end in view, according to the invention, said method is remarkable in that, upon the landing comprising a flare phase, when the aircraft is in approach of the runway, the following operations are performed on said aircraft:                anticipatively, a runway slope value is automatically transmitted to an automatic piloting system of the aircraft, and        said automatic piloting system uses such slope value to automatically manage the flare phase of the aircraft.        
Thus, thanks to the invention, thru the anticipated reception of the slope value of the landing runway, the automatic piloting system is in a position to anticipate enough of the particular characteristics, detailed hereunder, of the flare trajectory (which is significantly different either on a strongly climbing or strongly descending runway), and thus to automatically manage the flare phase and thus the landing of the aircraft.
Advantageously, said automatic piloting system can use the runway slope value anticipatively received:                to determine a flare starting height; and/or        to determine a nose up value of the aircraft upon flare; and/or        to determine a reference profile being adapted to an anticipated trajectory of the aircraft; and/or        to activate, upon the flare phase, extra aerodynamic surfaces being specific for the aircraft, essentially airbrakes.        
Furthermore, in a preferred embodiment, the runway slope value is determined, at least for a part of the runway (generally between the upstream threshold of the runway and the maximum spacing area of the wheel impact, for example 900 m downstream from said upstream threshold) at the level of which the flare must be performed, such slope value being then automatically transmitted to said automatic piloting system of the aircraft.
To do so, advantageously:                said runway slope value is manually input by an operator of the aircraft; and/or        said runway slope value is coming from an onboard data base; and/or        said runway slope value is automatically calculated from a runway profile coming from an onboard data base; and/or        said runway slope value is automatically measured thru at least one sensor arranged on the aircraft and making distance measurements ahead of the aircraft.        
It is also conceivable to combine several of the preceding methods to determine the runway slope value.
Moreover, in a preferred embodiment, a monitoring method is implemented so as to be able to detect an erroneous value for said slope value. Preferably, at least one of the following monitoring methods is implemented:                an auxiliary slope value is estimated based on the comparison between data supplied by a radio altimeter of the aircraft and a vertical inertial speed of the aircraft and such auxiliary slope value is compared with said slope value;        a correlation is made between a flied over airfield profile determined by a radio altimeter of the aircraft and an airfield profile stored in an onboard date base;        an auxiliary slope value is automatically measured on the aircraft thru an onboard sensor and such auxiliary slope value is compared which said slope value.        
Including for increased efficiency and reliability reasons, it is also conceivable to combine several of the preceding monitoring methods.
Advantageously, should an erroneous slope value be detected, at least one of the following operations is performed:                an alert is emitted;        it is asked to the aircraft crew, preferably via an alert, to implement a go-around;        the automatic piloting system is controlled so as to perform an automatic go-around;        the crew is provided with information on the origin of the failure and on actions to be taken, preferably thru a display.        
The present invention also relates to an automatic landing device of an aircraft on a landing runway with a strong slope.
According to the invention, said device is remarkable in that is comprises:                means to automatically transmit on an anticipated way the runway slope value to an automatic piloting system of the aircraft; and        said automatic piloting system uses said slope value to automatically manage the flare phase of the aircraft.        
In a particular embodiment, said device further comprises at least one of the following means:                at least one means to determine the runway slope value; and        at least one means to monitor said runway slope value.        
The present invention also relates to an aircraft, in particular a transport airplane, being provided with an automatic landing device such as the one above mentioned.